In display devices such as liquid crystal display devices, a touch panel equipped display device having a touch panel function has been developed. The touch panel equipped display device is capable of detecting a touch position (coordinates) when a panel surface thereof is touched by a subject to be detected (finger or input stylus).
Traditionally, as such a touch panel equipped display device (referred to as a display device below), a display device equipped with a touch panel of a so-called resistive type, capacitive type, or the like has been mainly used. However, this type of display device requires a special panel for position detection, for example, which causes the entire device to be thicker. Also, because the touch panel is disposed in a screen (display region) of the display device, a problem of lower visibility arises.
To address these problems, in recent years, a display device with built-in coordinate sensors, which has light-receiving elements (photo sensors) such as photodiodes or phototransistors incorporated in a screen of the display device, has been developed to replace the above-mentioned resistive or capacitive touch panel.
The display device with built-in optical coordinate sensors is described in Patent Document 1, for example.
FIG. 21 shows a schematic configuration of a liquid crystal display device described in Patent Document 1.
As shown in this figure, in the liquid crystal display device described in Patent Document 1, a distribution of received light intensities, which were changed as a result of a subject to be detected such as a finger blocking ambient light or as a result of the subject to be detected blocking or reflecting invisible light that was radiated to a liquid crystal panel 220 from a backlight 210, is detected by light-receiving elements 221 formed in the liquid crystal panel 220 in a matrix, thereby detecting the position (coordinates) of the subject to be detected. The backlight 210 has a light source unit 213 and a light guide plate 214, and the light source unit 213 has a visible light source 211 made of white LEDs (Light Emitting Diodes) and an infrared light source 212 made of infrared LEDs as an invisible light source. The light-receiving elements 221 are provided for respective elements (pixels) 222 with a ratio of one to one or one to several.